This has been screwed up since they moved away from the original, much MUCH nicer wacom digitizers(which were also more sensitive with the pen). I still haven't heard a legit explanation besides this other than "they're expensive"
One reason is that N-Trig works accurately right to the edge of the screen, but Wacom loses quite a lot of precision near the edge and pressure can behave differently there too. That's why Wacom's own Intuos and Cintiq devices have enormous bevels - the pen sensor continues out further.
